Code-Division Multiple Access (CDMA) is a well-known spread-spectrum physical layer technology for cellular systems. CDMA is currently used in the US IS-95 cellular standard and forms the foundation for some so-called 3rd generation cellular systems, such as IS-2000. Third generation (3G) systems are designed to support higher data rates and a wide range of new services. However, the demand for even higher data rates is expected to lead to the development of 4th generation systems in the years following the deployment of 3G.
There are several known techniques for increasing the peak per-user data rate in a CDMA system. First, the concept of multi-code CDMA involves simultaneously allocating multiple Walsh-code channels to a single user, thereby multiplying the peak data rate that can be delivered to any one user at a time. Second, increasing the chip rate, and therefore the system bandwidth, will directly increase the peak per-user data rate. Third, the use of higher-order modulation on each of the Walsh-code channels provides another method for increasing the data-carrying capacity.
Current cellular CDMA systems operate in relatively low bandwidths (1.25 MHz), and use low-order modulation with aggressive error control coding. On the forward link of these systems, the performance with a RAKE receiver is generally viewed as adequate for typical land-mobile cellular channels. However, if the previously mentioned techniques are used to significantly increase the data rate, the RAKE receiver can be shown to provide sub-optimal performance in severe multipath channels. Much of the reason for this sub-optimal performance can be traced to the fact that a multipath delay spread channel destroys the orthogonality of the Walsh-code channels. Walsh codes are orthogonal to each other, but have nonzero autocorrelation and nonzero cross-correlation properties. This loss of Walsh code orthogonality causes the output of a particular RAKE finger to contain significant energy (i.e., interference) from the multipath components having different arrival times. This interference, from other multipath components, is called intracellular interference. Intracellular interference becomes more severe as the channel multipath delay spread increases. As the system bandwidth is increased, the chip-span of a given multipath delay spread channel increases proportionally, which increases the required number of RAKE fingers in the RAKE receiver. As the number of multipath components resolved in the receiver increases, intracellular interference increases, which can further degrade RAKE performance.
As an alternative to a RAKE receiver, time-domain adaptive equalization is a technique for suppressing forward link intracellular interference caused by loss of Walsh code orthogonality. In contrast to a RAKE receiver, a time-domain MMSE adaptive equalizer balances the need to restore Walsh channel orthogonality with noise enhancement, thereby reducing the intracellular interference on the CDMA forward link. However, as system bandwidth increases, time-domain techniques known in the art for MMSE adaptive equalization have a complexity that grows rapidly with the channel length. A larger signal bandwidth typically results in a longer channel length, as measured in chip-times. As higher and higher system bandwidths are employed in future CDMA systems, equalization strategies having complexities lower than the time-domain techniques for MMSE adaptive equalization will be needed.
Therefore, it would be desirable to have a method and device for providing improved transmission format and equalization strategies to enable CDMA systems to support much higher data rates in future broadband wireless systems. Further, it would be desirable that the improved equalization strategies provide for the complexities and problems mentioned.